An active system, such as a biquad system, is known in the art for constituting analog filters. An analog filter made according to the biquad system is suitabe for use in an integrated circuit (referred to as IC hereafter). The biquad filter is used to obtain a desired filter characteristic by forming a circuit which realizes the secondary transfer function without using inductors but instead using an amplifier having a feedback loop.
Such an analog filter, e.g., a biquad filter, is widely used in color television fields, for instance, in a SECAM system color television system, in the form of a bell filter. The bell filter has a transfer function expressed by the following equation: ##EQU1## where, .omega.0 represents a natural angle frequency (e.g., 4.286 MHz);
H represents a gain coefficient (actual number); PA1 S represents a complex number; PA1 Qp represents a pole selectivity or a selectivity assigned for the denominator of the equation (1); and
Qn represents a zero selectivity or a selectivity assigned for the numerator of the equation (1).
When assuming the gain G at the natural angle frequency of 0 dB, the gain coefficient H becomes Qn/Qp.
To embody the transfer function various biquad filters have been considered. For instance, Y. Yamamoto et al., IEEE Transactions on Consumer Electronics, Vol. 34, No. 3, August 1988, pp 443-451 shows show such a biquad filter (see FIGS. 3 and 5) and a video processor for color TV (see FIG. 2) using such a filter.
However, such a conventional biquad analog filter has problems as follows. The filter can not totally complete a differential operation. Furthermore, the filter is influenced by a parasitic capacitance when it is implemented on an IC device.
Additionally, the filter requires an impedance transformer for driving capacitors in integration circuits constituting the filter.